Added support for the Postcode.nl Signal API, minor address validation features.

* PostcodeNl_Api_Helper_Data refactored for better testability
* HTTP status code considered more for REST api responses
* Added hook for Order creation to do a Signal check (with admin setting off by default)
* Updated composer & Magento package description
* Support for storing housenumber addition in third street field
* Version upped to v1.2.0.0
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